The thermal power and the power distribution of a nuclear boiling water reactor with natural circulation, such as the Dodewaard reactor in the Netherlands, is controlled with neutron-absorbing control rods that are moved stepwise into and out of the core of the reactor. To compensate for fuel depletion while operating the reactor at full power, some control rods are quite frequently moved to new axial positions. Toward the end of a power cycle, all the control rods are gradually withdrawn from the core.
The nuclear reactor is contained in a reactor vessel largely filled with water (see FIG. 1). During normal operation, much of the water is in the boiling phase, due to the nuclear heat produced in the reactor core. The uppermost part of the reactor vessel is filled with steam. This steam is fed to a turbogenerator in order to generate electricity. The water in the vessel is partly in gas phase and partly in liquid phase, its gas/liquid ratio depending on the thermal energy produced by the core of the reactor.
The interface between the steam and the boiling water, referred to as "froth level" in FIG. 1, is usually maintained at a constant position in the upper part of the vessel, by returning the condensed steam from the turbine condenser, referred to as "feed water" in FIG. 1, back to the reactor vessel with powerful high-pressure pumps.
No previous means have been capable of accurately detecting the position of the turbulent "froth level". A standpipe is normally connected on the outside of the vessel in which, on the principle of communicating vessels, a solid column of water is automatically created and in which the weight of the column is a measure of the position of the so-called "effective level" in the vessel. Pressure transducers connected to these standpipes provide level signals to the automatic control system to maintain the water level in the vessel at a fixed preset position. The reactor operators are able to manually change the set-point of this level control system.
However, during operation at high pressure and temperature, these generally applied pressure difference measuring systems suffer from decalibration effects. Moreover, during start-up and shutdown operations of the reactor system, this level measuring method is quite inaccurate, largely due to changing temperatures inside the vessel itself, as well as inside the standpipes located on the outside of the vessel.